A polymer electrolyte fuel cell has a solid polymer electrolyte sandwiched between an anode and a cathode. A fuel is fed to the anode, and oxygen or air is fed to the cathode, whereby oxygen is reduced at the cathode to produce electricity. The fuel is primarily hydrogen, methanol or the like.
To increase the reaction rate in the fuel cell and enhance the energy conversion efficiency of the fuel cell, a layer containing a catalyst (hereinafter, also referred to as a “fuel cell catalyst layer”) is conventionally provided on the surface of a cathode (an air electrode) or an anode (a fuel electrode) of the fuel cell.
As such a catalyst, noble metals are generally used. Of them, noble metals which are stable at high potential and have high catalytic activity, such as platinum or palladium, have been primarily used. However, since these noble metals are expensive and limited in resource amount, alternative catalysts have been desired.
Further, the noble metals used on the cathode surface are sometimes dissolved in an acidic atmosphere and are not suited in applications requiring long-term durability. Accordingly, it has been strongly demanded that catalysts are developed which are not corroded in an acidic atmosphere and have excellent durability and high oxygen reducing activity.
As a catalyst alternative to noble metals, those entirely free of noble metals, such as base metal carbides, base metal oxides, base metal oxycarbonitrides, chalcogen compounds and carbon catalysts, have been reported (for example, see Patent Literature 1 to Patent Literature 4). These materials are inexpensive and exist abundantly as compared with noble metal materials such as platinum.
However, catalysts containing base metal materials described in Patent Literature 1 and Patent Literature 2 have a problem in terms of their failure to provide oxygen reducing activity that is sufficient on a practical basis.
Catalysts described in Patent Literature 3 and Patent Literature 4, although showing high oxygen reducing catalytic activity, have a problem in terms of their extremely low stability under fuel cell operation conditions.
As a catalyst alternative to noble metals, Nb oxycarbonitrides and Ti oxycarbonitrides disclosed in Patent Literature 5 and Patent Literature 6 efficiently show the above performance and thus have been attracting particular attention.
Although the catalysts described in Patent Literature 5 and Patent Literature 6 have extremely high performance as compared with conventional catalysts alternative to noble metals, the production process thereof needs to include heating treatment under a high temperature of from 1600 to 1800° C. (for example, Example 1 of Patent Literature 5 or Example 1 of Patent Literature 6).
Performing such high-temperature heating treatment is not impossible on an industrial basis, but involves difficulty and invites increase in equipment cost and difficulty in operation control, leading to the increase in the production cost. Thus, the development of a process that achieves production at a lower cost has been desired.
Patent Literature 7 reports a technique relating to the production of a carbon-containing titanium oxynitride that contains carbon, nitrogen and oxygen.
However, according to the production process described in Patent Literature 7, the production of the carbon-containing titanium oxynitride requires two-stage synthesis: the preparation of a titanium oxynitride by reacting a nitrogen-containing organic compound with a titanium precursor, and the preparation of a carbon-containing titanium oxynitride by reacting a phenol resin with the titanium oxynitride precursor, and thus involves complicated steps. In particular, the preparation of the titanium oxynitride precursor requires complicated steps including stirring, heating and refluxing at 80° C. as well as cooling and concentrating under reduced pressure, thus resulting in high cost.
In addition, since the phenol resin is a thermosetting resin having a three-dimensional network structure, it is difficult to homogenously mix and react the phenol resin with a metal oxide. In particular, since the thermal decomposition temperature of the phenol resin ranges from 400 to 900° C., at a temperature of not higher than 1000° C., the carbonization reaction is unlikely to take place due to the complete decomposition of the phenol resin.
Patent Literature 7 and Non-Patent Literature 1 only describe applications to a thin film for a solar energy collector and a photocatalyst, failing to disclose or study a process for producing a metal oxycarbonitride having particulate or fibrous shape that is highly useful as an electrode catalyst and an application thereof.
Patent Literature 8 discloses a process for producing electrode catalyst characterized by calcining a mixed material of an oxide and a carbon material precursor. The production process, however, cannot provide an electrode catalyst having sufficient catalytic performance.
Patent Literature 9 discloses a fuel cell electrode catalyst obtained by using a polynuclear complex such as cobalt. This catalyst, however, has problems in terms of highly toxicity of the raw material, high cost and insufficient catalytic activity.
Non-Patent Literature 2 discloses a process for producing electrode catalyst characterized by calcining a mixed material of a titanium alkoxide and a carbon material precursor. The production process, however, does not use a nitrogen-containing organic substance and thus cannot provide an electrode catalyst having sufficient catalytic performance.